When the wind is drifting in their direction, and ideally if they have the washing hung out; burn them!
Seriously, go and see them and warn them someone is dumping tyres in the neighbourhood, ask if they have found any tyres are car parts in unusual places, tell them you are warning all the neighbours to be on the lookout, tell them you have reported it to the local council and the police, and that you are posting messages on the world wide web, act a little irrational like its some sort of major incident, and you are thinking of installing cctv. If it was them they will think you are a loony and keep well away.